During the radioactive transformations of uranium, radon-222 is generated. Radon-222 is a radioactive gas with a half-life of 3.825 days and emits alpha particles with energies of 5.0 and 5.5 MeV (Megaelectronvolts). It is a member of the series which begins with uranium and ends with lead-206. Until radon-222 disintegrates, it can migrate in the ground and through ore bodies because of diffusion and other transport phenomena for distances between a few inches to hundreds of feet. Therefore, during uranium exploration alpha particle measurements are an accepted method for measuring concentrations of radon-222 because they occur in the vicinity of uranium ore deposits. Generally, it is sufficient to measure the concentration of the alpha particles to indicate uranium deposits (see U.S. Pat. No. 3,665,194).
Several sensor types such as scintillation counters (U.S. Pat. Nos. 3,415,989 and 3,541,311) gas ionization counters and film or track registration means U.S. Pat. No. (3,655,194) which is sensitive to ionizing radiation can be used. Often, measurements requiring several days have to be performed in many locations. Because of the low cost, film is often used during uranium exploration. Each measurement costs approximately $15.00 at this time. This method has the disadvantage that the measurement results are not available until the film is recovered, developed, and the tracks caused by ionizing radiation are counted and plotted. Such film evaluation is usually performed at a special facility.
In the past, cost and power consumption of electronic particle counters have been prohibitive for so many long term applications. As the availability for inexpensive electronic calculators and electronic wrist watches indicates, small low cost electronic instruments with low power consumption have now become practical when demand for significant number of units is created. The cost of spectroscopic grade semiconductor radiation detectors is currently above $100.00. However, for the proposed instrument as described herein, detectors can be used which do not meet all the requirements of the spectroscopic grade detectors in terms of collection efficiency, current leakage, etc. and, therefore, the cost per detector can be kept to about one-half. The suitability of such detectors for the purposes of this invention, has been determined with a polonium alpha particle source. High voltage biasing, e.g., on the order of 60 volts, is not needed.
Solid-state detectors and especially surface barrier detectors (U.S. Pat. No. 2,670,441) can be degraded or even destroyed because of high humidity. Attempts to seal the detector with a thin (.0003 inch thick) foil are not often successful for long periods because the seals degrade or the very thin foil has pin holes and ambient gas enters the volume of gas surrounding the sensitive detector area. Moreover, condensation of water vapor in the apparatus interferes with alpha particle measurements. These semiconductors are sensitive to light and for accuracy, visible light radiation must be excluded.
The present invention provides means for avoiding problems attendant high humidity and water vapor condensation which is encountered in ground measurements.
Another aspect of the present invention is that instead of using relatively large area semiconductors of high resolution power which under current conditions cost approximately $200.00 each, smaller size, less expensive semiconductors, can be used very effectively for the counting purposes of the present invention. These devices cost considerably less, for example, approximately $40.00 each. In the present embodiments, noise is unimportant and the cost of higher grade semiconductors and associated circuitry can be avoided without sacrifice of satisfactory performance for radon-222 detection purposes.
Still further, it has been found possible to place within the container which is, for example, an aluminum tube 11/2 inches diameter by about 11 to 12 inches long, a light emitting diode (LED) numerical display enabling a direct reading of the accumulated count at the location of the device. Alternatively, the impulses resulting in such a display can be transmitted to a remote receiving station by means of a cable containing a suitable number of leads, e.g., 16 leads.
By "shaped desiccant" as used herein is meant a desiccant having a predetermined physical configuration. It may be formed by compacting a body of powdered desiccant under high pressure into a given shape, e.g., a ring shape, or depositing the desiccant on a matrix or support.